In an internal combustion engine such as diesel engines, a part of gasoline engines and the like, an NOx purification catalyst (deNOx catalyst) and a DPF (Diesel Particulate Filter) are used in order to reduce NOx (nitrogen oxide) and PM (particulate matter) in an exhaust gas.
The NOx purification catalysts include a lean NOx trap catalyst (LNT catalyst), a selective reduction catalyst (SCR catalyst) and the like, and one of the lean NOx trap catalysts is an NOx occlusion reduction catalyst. The NOx occlusion reduction catalyst is a catalyst carrying an NOx occlusion material occluding NOx and precious metal, and if an air/fuel ratio of an inflow exhaust gas is in a lean (hyperoxia) state and oxygen (O2) concentration in an atmosphere is high, nitric oxide (NO) in the exhaust gas is oxidized on the precious metal so as to become nitrogen dioxide (NO2), and the nitrogen dioxide is combined with the NOx occlusion material such as barium (Ba) so as to become a nitrate (Ba2NO4) and the like and to be occluded.
Moreover, if the air/fuel ratio of the exhaust gas flowing into the NOx occlusion reduction catalyst becomes a theoretical air/fuel ratio or enters a rich (low oxygen concentration) state and the oxygen concentration in the atmosphere lowers, the NOx occlusion material is combined with carbon monoxide (CO), the nitrate is dissolved, and nitrogen dioxide is emitted. The emitted nitrogen dioxide is reduced by unburned hydrocarbon (HC), carbon monoxide and the like contained in the exhaust gas by a three-way function of the precious metal so as to become nitrogen (N2), while components in the exhaust gas are emitted into the atmospheric air as harmless substances such as carbon dioxide (CO2), water (H2O), and nitrogen.
Thus, in the exhaust gas purification system including the NOx occlusion reduction catalyst, when an NOx occlusion capability gets closer to saturation, control in which the air/fuel ratio of the exhaust gas is brought into a rich state so as to lower the oxygen concentration of the inflow exhaust gas, that is, rich control for recovering the NOx occlusion capability is executed so that an NOx regeneration operation for emitting the adsorbed NOx and reducing the emitted NOx to nitrogen by the precious metal is performed.
Since catalysts including the NOx occlusion reduction catalyst need to be at an activation temperature or more in general so as to activate its catalytic action, a catalytic reaction is not promoted at a low temperature at which the catalyst is not activated, which results in a problem that NOx purification catalysts such as the NOx occlusion reduction catalyst and a selective reduction catalyst has a low NOx purification rate.
In order to improve the NOx purification rate in the catalyst at a low temperature, temperature rise control of the exhaust gas is examined also in exhaust gas treatment of a diesel engine. The temperature rise control is control for raising an exhaust gas temperature by multistage temperature rise injection (multi injection) which is a combination of pre injection (pilot injection), main injection, after injection and the like so as to keep a combustion temperature in a cylinder (in-cylinder) of an expansion stroke at a top dead center and after of a piston at a high temperature. By means of the temperature rise control, the exhaust gas temperature can be raised in an early stage, and activation of the catalyst of the exhaust gas purification device disposed in an exhaust passage can be expedited.
Moreover, in the exhaust gas purification system including a DPF for collecting PM in the exhaust gas, when a PM collection amount gets closer to an amount that can be collected by the DPF (an amount in a saturated state), PM regeneration control in which a temperature of the DPF is raised to a temperature at which the collected PM starts combustion or more in order to recover the PM collection capability and to burn/remove the PM is executed. In the temperature rise of the DPF, temperature rise control in which the temperature of the exhaust gas flowing into the DPF is raised so as to raise the temperature of the DPF by the exhaust gas at a high temperature is executed.
However, in the temperature rise control of the exhaust gas during cooling, most of the fuel injected into the cylinder by the temperature rise control of the multistage temperature rise injection is burned in the cylinder, but unburned hydrocarbon (HC) passes through the exhaust gas purification device as it is in some cases. That is, when the temperature rise control of the exhaust gas is executed, if the temperature of the catalyst provided in the exhaust passage is at a catalyst activation temperature or less, there is a problem that the unburned hydrocarbon discharged from the cylinder reaches a tail pipe in a last part of the exhaust passage and flows out (slip) into the air and is emitted into the air.
The HC slip is likely to occur not only during cooling in the NOx purification catalyst and NOx regeneration in the NOx occlusion reduction catalyst but also in PM regeneration for burning/removing the PM in the DPF at a low load of the engine.
For example, as described in Japanese patent application Kokai publication No. 2010-31833, in order to reduce emission of the HC (hydrocarbon) into the air, in an exhaust gas purification device with a configuration of a pre-stage oxidation catalyst (DOC) and a DPF for performing DOC temperature rise by early post injection and DPF temperature rise by oxidation reaction heat in DOC of an unburned component supplied by late post injection, an exhaust gas purification device of a diesel engine for suppressing rapid rise of an injection amount of the late post injection and preventing the HC slip by giving a certain period of time of delay to a target late post injection amount is proposed.
However, when such DOC is used, the DOC for oxidizing the unburned component (unburned hydrocarbon) in the exhaust gas needs to cover the entire operation region of the internal combustion engine and thus, size reduction is difficult, and there is limitation in installation close to the engine body. Moreover, if exhaust pipe direct injection is used, since a certain period of time is needed for evaporation of the injected fuel, and if an interval between an HC injection valve nozzle (exhaust pipe direct injection device) and the DOC is small, soot is generated by partial oxidation (cracking) of the fuel on a front surface of the DOC and blocking might be caused and thus, the interval between the HC injection nozzle and the DOC needs to be taken as large as 1 m or more, for example.
For these reasons, too, the DOC cannot be arranged close to the engine body and thus, the temperature falls due to heat radiation before the exhaust gas discharged from inside the cylinder reaches the DOC. Thus, at a low temperature, the unburned HC in the exhaust gas cannot be sufficiently oxidized by the DOC, and HC slip will occur.
As a measure against the HC slip at a low temperature, as described in Japanese patent application Kokai publication No. 2011-163250, for example, in an exhaust gas purification device with a configuration of the pre-stage oxidation catalyst (DOC) and a DPF for performing the DOC temperature rise by early post injection and the DPF temperature rise by oxidation reaction heat in DOC of an unburned component supplied by late post injection, in order to prevent emission of the unburned HC component in large quantity to an outside until the DOC reaches the active temperature after the early post injection, an exhaust gas treatment method and device of an internal combustion engine for promoting activation of the DOC and for reducing the HC slip by increasing a load of a diesel engine through an increase in power of an oil circulation pump and increasing an exhaust gas temperature rise gradient in a DOC temperature rise stage until start of the early post injection is proposed. However, with this exhaust gas treatment method and device, the load of the diesel engine is increased in order to raise the temperature in the cylinder of the internal combustion engine at low-temperature start, and thus there is a concern of deterioration of fuel consumption.